The reason was not tied to allegations of widespread intimidation by drug gangs or other corruption.

The reason was because Mexican boxer Juan Manuel Marquez, in a major bout with Philippine fighter Manny Pacquiao in Las Vegas in November, wore trunks emblazoned with the emblem of Mexico's resurgent Institutional Revolutionary Party, or PRI.

He lost, but PRI won

The welterweight match was broadcast worldwide - the day before the vote in Michoacan. The PRI won in Morelia and almost every­where else in Michoacan - an election seen as crucial because it was the last statewide vote before next year's presidential race, and a test of the PRI's comeback.

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Mexico's highest electoral tribunal said the logo, along with an appearance by a PRI candidate at a rally, violated rules prohibiting partisan publicity in the final days of the campaign.

Calderon's conservative National Action Party, or PAN, celebrated the ruling and said it hoped results for the Michoacan governorship would also be annulled. Calderon's sister ran for that post and narrowly lost to the PRI candidate.

The court did not immediately release an explanation of the reasoning behind its decision, which requires that the Morelia race be run again on an as-yet-undetermined date in the next five months.

Cartel meddling cited

The PAN and the leftist Democratic Revolution Party had also complained that Michoacan's powerful drug cartels had intimidated voters on behalf of the PRI. On the eve of the elections, one local cartel placed a notice in a newspaper in the town of La Piedad threatening citizens who voted for the PAN.

Calderon has said in several public addresses that organized crime helped defeat his sister, Luisa Maria Calderon, and has described drug cartel meddling in elections as a potential national menace.

PAN sees another win

In addition, the PAN said in its complaint about the Morelia race that winning gubernatorial candidate Fausto Vallejo and Morelia mayoral candidate Wilfrido Lazaro had made unauthorized appearances on television to announce the end of their campaigns.

A high-ranking PAN official, Juan Molinar Horca­sitas, said the party had made the same arguments in its appeal of the governor's race, and expected another victory in court.

"According to the law, in the same circumstances the same decision should be taken," he said. "It seems to me that, as a result, the electoral tribunal should annul the governor's race."

A new vote would be almost certain to draw in the national resources of the governing party, the PRI and the PRD in what could become a proxy presidential race.

Marquez lost the Nov. 12 bout, which was viewed live by millions in Mexico. Neither he nor his managers could be reached for comment.